The present invention relates to control mechanisms for propeller driven apparatus, such as turbines, motors or generators, extracting power from a fluid stream. Particular applications include ram air turbines (RATs) mounted on aircraft wing-borne refueling pods.
Propeller driven turbines, for example, are typically placed in a fluid stream such that the flow of fluid causes the propellers to rotate. That rotation can be used to generate mechanical, hydraulic or electric power for a variety of purposes. It is often desirable in such applications to control the starting and stopping of propeller rotation without withdrawing the apparatus from the fluid stream.
Various control mechanisms have been suggested for use with such apparatus. In certain applications, such as airborne power sources, performance of many control mechanisms have been less than desired. For example, with RATs used on aircraft refueling pods the control mechanisms are subject to significant environmental extremes as a result of altitude variation and local climate. Prior devices to control stopping and restarting of airborne RATs have been found to be insufficiently reliable in several respects. First, the restart mechanisms were not sufficiently energetic to consistently assure restart on command and reliable speed control over long periods of use. In part, this is believed to be because of weight constraints and in part because more energetic restart mechanisms applied more destructure stresses and wear to components which would result in either higher material costs and/or greater instances of repair.
Also, prior devices tended to be overly complicated and have a high number of components (with the resulting tight tolerance constraints), thus also increasing the costs of manufacture and repair. In addition, prior devices had an unacceptable number of hazardous failure modes. Further, some prior devices included control mechanisms which did not fully feather the blades during nonuse, thus creating increased aerodynamic drag in the airstream.
Accordingly, it is an object of the present invention to provide an improved control mechanism for propeller driven devices, such as RATs. Other objects include the provision of a robust control mechanism that:
1. permits reliable and consistent stopping and restarting in the fluid stream, PA1 2. provides reliable, safe speed control during start-up and normal operation; PA1 3. has a minimal number of parts and is relatively simple to manufacture and/or repair, PA1 4. is minimally affected by use in a hostile environment, PA1 5. has minimal hazardous failure modes, PA1 6. provides a large safety factor on useful life, and PA1 7. avoids dependency on tight tolerances.
These and other objects are attained by the provision of a control mechanism which feathers the propeller blade pitch during stopping and stores restart energy in the blade pitch governor mechanism. Deceleration to the feathered mode is aided by the airstream. During restart, excess stored energy is used to increase reliability, but is then dissipated mechanically, as by a flywheel mechanism, to maintain low internal impact stresses. More specifically, where single governor spring is used for controlling the pitch of all propeller blades, a braked ball screw device is used to compress that spring during stoppage. Upon restart, the governor spring expands and causes blade pitch change. The ball screw arrangement then dissipates a large portion of the excess energy through torque transmission to mechanical components.
Other objects, advantages and novel features of the present invention will now become readily apparent to those of skill in the art from the following drawings and description of preferred embodiments.